i ' BYZANTINE ARCHITECTURE . 15
coalesce and the arches connecting the columns with the walls cut into the stilted part of the dome arches, with the result that all the structural arches and vaults spring from the same level
Arches.—Though the pointed arch was known and employed in cisterns, as in the Cistern of the One Thousand and One Columns, Bin-bir-derek, the circular arch is invariably found in work meant to be seen* The difficulty attending this form, in which arches of unequal breadth do not rise to the same height, was overcome, as in the West, by stilting, that is, by raising the smaller arches on straight c legs * to the required height. The stilted arch, indeed, seems to have been admired for its own sake, as we find it used almost universally both in vaulting and in decorative arches even where it was not structurally required. In windows and in the arches connecting the dome columns to the wall stilting is sometimes carried to extremes.
Domes.—The eastern dome of S. Irene, erected about 740 A.D., is generally considered to be the first example of a dome built on a high drum, though S. Sophia of Salonica, an earlier structure, has a low imperfect drum. After this date the characteristics of the Byzantine dome are the high drum divided by ribs or hollowMegments on the interior, polygonal on the exterior, and crowned by a cornice which is arched over the windows.1
Drumless domes are sometimes found in the later churches, as in the narthexes of the Panachrantos and S. Andrew, the angle domes of S. Theodosia, and in Bogdan Serai. These are ribless hemispherical domes of the type shown in Fig. 8, and are in all cases without windows. The earlier system of piercing windows through the dome does not occur in the later churches, though characteristic of Turkish work.
The three diagrams (Figs. 8, 9, and 10) illustrate the development of the dome : firstly, the low saucer dome or dome-vault in which dome and pendentives are part of the
1 Strzygowski's views as to the early date of the drum-dome are not universally accepted. The examples he produces seem rather octagons carried up from the ground to give a clearstory under the dome than true drums interposed between the dome and its pendentives.